1. Field Of the Invention
The present invention relates to phase locked loops comprising linear phase detectors/comparators in general and in particular to a phase locked loop comprising a linear phase comparator with a loop gain stabilization circuit for providing a relatively constant loop gain in such circuits over a wide range of frequencies.
2. Description of the Related Art
A linear phase comparator in the form of a harmonic sampler can be used advantageously in place of a conventional digital detector in a phase locked loop for dividing and multiplying microwave frequencies in ultra low phase noise microwave frequency synthesizers and other microwave circuits so long as a means, i.e. a frequency presteer circuit, is provided for setting the initial nominal frequency of the voltage controlled oscillator (VCO) in the loop. For example, in applicant's U.S. patent application, Ser. No. 08/060,755, filed May 12, 1993, entitled ULTRA LOW PHASE NOISE MICROWAVE SYNTHESIZER, and in applicant's U.S. patent application, Ser. No. 08/051,624, filed Apr. 22, 1993, entitled ULTRA LOW NOISE FREQUENCY DIVIDER/MULTIPLIER there are disclosed frequency multiplier and divider circuits comprising harmonic samplers/presteer circuits in otherwise conventional phase locked loops.
As discussed in applicant's above-identified patent applications, the contents of which are included herein by reference, the intermediate frequency (F.sub.if) output of a harmonic sampler becomes a direct current (dc) level, i.e. null, whenever the frequency F.sub.rf of the high frequency input signal of the sampler is a harmonic of the sampling frequency F.sub.s. For this reason a presteer circuit is employed to drive the VCO to a selected initial nominal frequency. Once the presteer circuit has driven the VCO to the selected nominal frequency, i.e. selected harmonic of the sampler, the output of the presteer circuit is disabled and the normal output of the sampler takes over and controls the frequency of the VCO.
In most, if not all, phase locked loops it is important that the loop gain of the phase locked loop remain relatively stable. As will be appreciated, whenever the loop gain is not constant over a broad operating frequency range of a VCO, the frequency of the VCO becomes an unpredictable function of the phase difference between the frequency of the VCO and a reference frequency, e.g. the sampling frequency, of a harmonic sampler.